The present disclosure relates, in various embodiments, to compositions and processes suitable for use in electronic devices, such as thin film transistors (“TFT”s). The present disclosure also relates to components or layers produced using such compositions and processes, as well as electronic devices containing such materials.
Thin film transistors (TFTs) are fundamental components in modern-age electronics, including, for example, sensors, image scanners, and electronic display devices. TFT circuits using current mainstream silicon technology may be too costly for some applications, particularly for large-area electronic devices such as backplane switching circuits for displays (e.g., active matrix liquid crystal monitors or televisions) where high switching speeds are not essential. The high costs of silicon-based TFT circuits are primarily due to the use of capital-intensive silicon manufacturing facilities as well as complex high-temperature, high-vacuum photolithographic fabrication processes under strictly controlled environments. It is generally desired to make TFTs which have not only much lower manufacturing costs, but also appealing mechanical properties such as being physically compact, lightweight, and flexible. Organic thin film transistors (OTFTs) may be suited for those applications not needing high switching speeds or high densities.
TFTs are generally composed of a supporting substrate, three electrically conductive electrodes (gate, source and drain electrodes), a channel semiconducting layer, and an electrically insulating gate dielectric layer separating the gate electrode from the semiconducting layer.
It is desirable to improve the performance of known TFTs. Performance can be measured by at least three properties: the mobility, current on/off ratio, and threshold voltage. The mobility is measured in units of cm2/V·sec; higher mobility is desired. A higher current on/off ratio is desired. Threshold voltage relates to the bias voltage needed to be applied to the gate electrode in order to allow current to flow. Generally, a threshold voltage as close to zero (0) as possible is desired.
While p-type semiconducting materials have been extensively researched, less emphasis has been applied to n-type semiconducting materials. N-type organic semiconductors having high electron mobility and stability in air, especially solution processable n-type semiconductors, are rare due to their air sensitivity and difficulties in synthesis compared to p-type semiconductors. Because n-type semiconductors transport electrons instead of holes, they require a low Lowest Unoccupied Molecular Orbital (LUMO) energy level. To achieve low LUMO levels, electron-withdrawing groups such as fluoroalkyl, cyano, acyl, or imide groups have been applied to some n-type organic semiconductors. However these electron-withdrawing groups can only be used as substituents or sidechains on conjugated cores such as acenes, phthalocyanines, and oligothiophenes, and cannot be used as conjugated divalent linkages themselves for constructing linear n-type semiconducting polymers. Most reported high-mobility air-stable n-type semiconductors are small molecular compounds and can only be processed using expensive vacuum deposition techniques to achieve maximum performance.